Bone, Cartilage and Joints 2

Describe the structure of compact bone, discuss these features:

• The periosteum and endosteum

• Volksmann’s canals and haversian canals

The Periosteum and Endosteum

• The periosteum is a membrane covering the outer surface of bones, providing nerves and blood supply and containing cells for bone growth and repair. In contrast, the endosteum is a membrane that lines the inner surfaces of bones, such as the medullary cavity and spongy bone spaces, also containing bone cells involved in development and remodeling

•  Contain osteogenic (stem) cells differentiate to Osteoblasts and produce Osteoid

•  Gives rise to outer/inner circumferential lamellae of bone (bony layers)

Volkmann's canals and Haversian canals

• Blood vessels penetrate inwards via Volkmann and Haversian canals

• Horizontal = volkmann

• Vertical = haversian

• Osteogenitor cells associated these canals

How and where do the Haversian systems or Osteons develop?

Within the diaphysis, Haversian systems or Osteons develop around blood vessels and align parallel to the long axis of the bone.

• Osteoblasts form concentric lamellae (4-20) depending on stage of growth

• Specific orientation of collagen fibres in each lamellae- impart great strength

• Constantly being remodelled and reformed

Interstitial lamella- represent remodelled osteons by osteoclasts, remnant haversian systems, fill in the gaps between newly formed osteons

What is an osteon?

Alternative name for haversian system

Where is cancellous (spongy or trabecular) bone found?

• What is the main function?

Found within marrow cavity

Consist of bony spicules/trabeculae covered with endosteum

Supports hematopoietic tissue

What two ways does bone develop? Which structures develop from each process?

1. Intra-membranous ossification

- Occurs in restricted areas of the embryo

- Formation of the flat bones of skull, mandible and maxilla

- Forms directly from osteogenic connective tissue

• No pre-existing hyaline cartilage model

2. Endochondral ossification

- Form of ossification where a cartilage model formed first then replaced by bone

- Growth of long bones relies on this process continuing to adult hood at epiphyseal growth plate

• Continues during puberty

What forms directly from mesenchymal tissue? (Additionally what is mesenchymal tissue?)

BONE forms directly from a primitive connective tissues called mesenchymal tissues when considering Intra-membranous ossification

• Fibrocytes in this tissue get a signal to change into osteoblasts, to lay down osteoids

Summarize the stages of endochrondral ossification. 

• Most bones form via endochondral ossification

Endochondral ossification, usually occurs from the middle → Outwards

• Mesenchymal condensation

• Differentiation into hyaline cartilage (The cartilage model)

• Vascular invasion & mineralisation

• Calcification (Which causes it to harden)

• Ossification centres

What structure allows long bones to continue growth?

the growth plate

Where is the epiphyseal growth plate located?

Epiphyseal growth plate -hyaline cartilage located between the epiphysis and metaphysis

Describe the different zones in the epiphyseal growth plate.

Describe the arteries supplying blood to supply epiphyseal growth plate.

• branches of the epiphyseal artery supply the resting zones of the growth plate

• branches of the metaphyseal artery form capillary loops at the metaphyseal side where endochondral ossification occurs

• integrity of this vascular supply is critical to the process of endochondral ossification

What role do osteoclasts have in bone remodelling?

1. Activation

Osteoclast precursors are recruited to the haversian canal and differentiate into osteoclasts. Osteoclasts are lining the bone lamella facing the canal and start the bone resorption process of the inner lamella and consecutive lamellae toward the outer lamella. Interstitial lamellae are residuals of the remodeling osteon.

2. Resorption

Additional osteoclast precursors are recruited as lamellar resorption progresses slightly beyond the boundary of the original osteon. When osteoclasts stop removing bone, osteoblasts appear (osteoclast to osteoblast reversal).

3. Reversal

Osteoblasts reverse the resorption process by organizing a layer inside the reabsorption cavity and starting to secrete osteoid. The cement line indicates the boundary of the newly organized lamella. New bone lamellae continue to be deposited toward the center of the osteon.

4. Formation

Osteoblasts continue laying down bone and eventually become trapped within the mineralized bone matrix and become osteocytes. A new osteon or haversian system is formed. In osteoporosis, more bone is reabsorbed than is subsequently produced.

How does the remodeling of cancellous (spongy or trabecular bone) occur?

Trabecular bone remodeling occurs on the bone surface, in contrast to cortical bone remodeling, which occurs within an osteon. The trabecular endosteal surface is remodeled by this mechanism similar to cortical bone

• remodeling: osteoclasts create a resorption space limited by a cement line. Then osteoblast line the cement line surface and start to deposit osteoid until new bone closes the resorption space.

Endochrondral Ossification in more Detail:

• Describe the first stages of long bone growth forming the primary center of ossification.

1. Cartilage Model Formation

• Hyaline cartilage forms the future long bone.

• Chondrocytes proliferate and secrete cartilage matrix.

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2. Chondrocyte Hypertrophy & Signaling

• Central (diaphyseal) chondrocytes hypertrophy.

• Secrete type X collagen and VEGF to attract blood vessels.

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3. Matrix Calcification & Chondrocyte Death

• Hypertrophic chondrocytes promote calcification of the surrounding cartilage.

• Cells undergo apoptosis, leaving cavities in calcified cartilage.

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4. Vascular Invasion & Osteoclast Activity

• Blood vessels from the perichondrium invade the calcified cartilage.

• Osteoclasts resorb calcified cartilage remnants, clearing space for new bone.

• Osteoprogenitor cells arrive alongside osteoclasts.

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5. Osteoblast Activity & Bone Deposition

• Osteoprogenitors differentiate into osteoblasts.

• Osteoblasts deposit osteoid on the residual calcified cartilage → forms primary ossification center.

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6. Periosteal Collar Formation

• Perichondrial osteoblasts form a periosteal bone collar around the diaphysis, giving structural support.

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7. Growth Plate & Secondary Centers

• Ossification expands toward the epiphyses, leaving a growth plate for longitudinal growth.

• Secondary ossification centers form in epiphyses later.

Endochrondral Ossification in more Detail:

• Describe the lengthening stages of long bone growth involving the growth plate.

• Epiphyseal (growth) plate remains

  • Located between the diaphysis (shaft) and each epiphysis (end).

  • It’s a layer of hyaline cartilage that does not ossify immediately when the primary center forms.

  • This cartilage continues to grow interstitially (from within).

• Chondrocyte activity in the growth plate drives elongation
  The growth plate is organized into zones:

  • Resting zone: small, inactive chondrocytes.

  • Proliferation zone: chondrocytes divide rapidly, lengthening and swelling, stacking into columns → pushes epiphysis away from diaphysis.

  • Hypertrophy zone: chondrocytes enlarge, some begin to die

  • Calcification zone: matrix calcifies, chondrocytes die and are encased 

  • Resorption Zone:

  • Blood vessels and osteoblasts invade.

  • Osteoclasts remove calcified cartilage (reabsorb)

  • Ossification zone: Osteoblasts lay down new bone on scaffolding of calcified cartilage, penetrating with osteogenic cells and blood vessels , osteoclasts increase space for vessels to lay down matrix

    • This is woven bone - will eventually become compact bone

• Result

  • The epiphyses are pushed farther apart, lengthening the bone.

  • The diaphysis fills in with bone where cartilage is replaced.

  • As long as the growth plate remains active, the bone keeps elongating.

• Closure of the growth plate

  • Eventually, chondrocyte proliferation slows, cartilage is completely replaced by bone, and the epiphyseal plate becomes the epiphyseal line.

  • At this point, longitudinal growth stops.

Describe how the epiphyseal plate closes.

the union of the primary and secondary ossification centers corresponds to the closure of the growth plate.

• Primary ossification center – forms in the diaphysis (shaft) of the bone during fetal development.

  • This is where bone tissue first replaces cartilage in the middle of the long bone.

• Secondary ossification centers – form in the epiphyses (ends of the bone) after birth.

  • These create the bony epiphyses but leave a layer of cartilage between the epiphysis and diaphysis — the epiphyseal (growth) plate.

The epiphyseal plate is the site of interstitial growth — it allows the bone to lengthen during childhood and adolescence.
Bone continues to be deposited on the diaphyseal side while cartilage is generated on the epiphyseal side.

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• As skeletal maturity is reached, chondrocyte proliferation slows and stops.

• The remaining cartilage in the growth plate becomes ossified.

• When all cartilage is replaced by bone, the epiphysis and diaphysis are fully continuous — meaning the secondary and primary ossification centers have merged.